how come the ground under lakes and rivers don’t drink up all the water but plain surface ground drinks up everything eventually?
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New water keeps coming.
Everyone else seems to be missing the point. Yes, as the ground saturates the amount it absorbs becomes much much more slow. And that’s an important part of the puzzle.
But more importantly, the answer is that there’s enough flow in to counter the flow out (and when there isn’t, the lake doesn’t stick around for you to notice). If you just had a standing pond or lake, and it never rained, and there was no incoming river, the pond or lake would eventually empty. The ground wouldn’t just suck up 1 trillion gallons and then be permanently leave the rest in the lake. It would keep getting sucked up slowly until the lake emptied. In fact, this is what defines “plain surface ground”, it’s ground that doesn’t receive enough water to cancel out the amount it drinks up, so it usually doesn’t have standing water except temporarily during heavy rainfall.
But in real life, if you’re not in a desert, it keeps raining, which tends to accumulate in low parts of the terrain, which then form rivers and lakes and flows downstream, which then provides new flow of water to other areas. If the constant flow in of new water is enough to counter the amount that the ground drinks up, then you get a long term river or lake or marsh or something of that nature.
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